Processes for producing alkyl ester of fatty acid

ABSTRACT

The present invention relates to a process for preparing an alkyl ester of a fatty acid wherein a transesterification reaction is carried out between a fat or oil and an alcohol in the presence of a catalyst comprising a composite metal oxide having a perovskite structure; and a process for preparing an alkyl ester of a fatty acid wherein a transesterification reaction is carried out between a fat or oil and an alcohol in the presence of a catalyst comprising at least one member selected from the group consisting of oxides, hydroxides and carbonates of alkaline earth metals is used as the catalyst, with the alcohol made into a supercritical state or subcritical state. According to the present invention, the alkyl ester of a fatty acid which can be effectively utilized as a diesel fuel oil or the like can be prepared at high efficiency and on an industrial scale mainly from triglyceride contained in a fat or oil, especially a waste oil.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP02/03039 which has an Internationalfiling date of Mar. 28, 2002, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to a process for preparing an alkyl esterof a fatty acid.

BACKGROUND ART

Currently, in our country Japan, various edible oils have been used inlarge amounts. A part of used oil (waste edible oil) is reused as a rawmaterial for soap or the like. However, a majority of the used oil arenot collected, and are actually transported to refuse disposal, andincinerated together with burnable refuse, or reclaimed together withunburnable refuse.

On the other hand, there has been previously known that an alkyl esterof a fatty acid can be prepared by subjecting a monoglyceride, adiglyceride or a triglyceride, which is a main component in a vegetableoil, to transesterification reaction with an alkyl alcohol, therebygiving an alkyl ester of a fatty acid (for instance, “Yuki KagakuHandobukku (Organic Chemistry Handbook),” published by Gihodo, 1988, p.1407-1409). Also, by utilizing this reaction, various studies have beenmade on the techniques for preparing an alkyl ester that can be used asa diesel fuel oil from a vegetable fat or oil, a waste edible oil or thelike (for instance, Japanese Patent Laid-Open Nos. Hei 7-197047, Hei7-310090 and the like). In these techniques, alkyl esters which can meetthe requirement of the current Quality Assurance Regulation regarding agas oil have not been obtained.

Incidentally, since the transesterification reaction is an equilibriumreaction, the equilibrium is shifted to the product side by using analkyl alcohol, one of the raw materials, in a large amount, or removingglycerol produced as a side reaction product, thereby increasing anyield. Also, this reaction is said to be more advantageous in a vaporphase reaction than in a liquid phase reaction from the viewpoint ofstate of equilibrium. Further, in order to increase the reaction rate, acatalyst is generally utilized.

In the preparation of acetic acid, a higher fatty acid, an unsaturatedcarboxylic acid or the like, which is a representative industrialprocess of the transesterification reaction, an acidic catalyst isgenerally used in a large amount. For instance, a protonic acid such assulfuric acid or phosphoric acid has been used as an esterificationcatalyst for non-aromatic carboxylic acids, and boric acid or sulfuricacid has been used for an esterification of a phenolic acid. However,since these reactions are basically a homogeneous reaction system inwhich a catalyst exists in the dissolved state in a reaction solution,there has been a problem that the separation and collection of thecatalyst from the formed liquid are difficult.

A solid acidic catalyst is also well used. In the transesterificationreaction of terephthalic acid or methacrylic acid, there has been usedSO₄ ²⁻—TiO₂, TiO₂—SiO₂, Al₂(SO₄)₃/SiO₂.Al₂O₃, a sulfonic acid-basedion-exchange resin and the like. In addition, a heteropoly-acid is saidto be an excellent esterification catalyst. In a case where theheteropoly-acid is supported to SiO₂ or activated charcoal, theheteropoly-acid as a vapor phase catalyst has been known to exhibit anactivity which is higher than those of SiO₂—Al₂O₃ and the solidphosphoric acid. Further, a clay mineral has been also used as acatalyst. Since these solid acidic catalysts and mineral catalysts donot have to be separated from the formed liquid, use of these catalystsis more excellent from the viewpoint of simplification of the reactor.However, these industrial catalysts have a fatal defect that theactivity for the transesterification reaction of a fat or oil is low.Therefore, the above process has not yet been actually used on anindustrial scale to date.

As a technique for applying a solid acidic catalyst totransesterification of a fat or oil, there has been proposed a techniqueas disclosed in, for instance, Japanese Patent Laid-Open No. Hei6-313188. Moreover, the catalyst used in this technique includes asimple or composite metal oxide, a metal sulfate, a metal phosphate, animmobilized acid in which the acid is supported or immobilized to acarrier, a natural mineral and a layered compound, a solidheteropoly-acid, a superacid, a synthetic zeolite, an ion-exchange resinand the like. However, in this technique, the catalytic activity for thetransesterification reaction of a fat or oil is as low as that of theconventional processes described above. Therefore, in order to achieve ahigh yield, there has been necessitated that a ratio of the solid acidiccatalyst is increased in the reaction system or that the reaction timeis lengthened.

A basic catalyst has been also used in the transesterification reaction,and there has been known that a metal alcoholate is effective as thisbasic catalyst. Therefore, sodium alcoholate or potassium alcoholate hasbeen generally used as the metal alcoholate. Also, as the basiccatalyst, sodium hydroxide, potassium hydroxide, sodium carbonate or thelike has been used. These exhibit high activity for thetransesterification reaction of a fat or oil. However, the conventionalbasic catalyst acts in a dissolved state in a reaction solution in thesame manner as the acidic catalyst mentioned above. Therefore, the basiccatalyst dissolves in the formed liquid, so that the problem that itsseparation and collection are difficult has not been eliminated.

In addition, there has been attempted to use a solid basic catalyst inthe transesterification reaction, and as such a solid basic catalyst,there has been proposed an ion-exchange resin having an amine-based base(for instance, Japanese Patent Laid-Open No. Sho 62-218495). In thistechnique, the problem of separating and collecting the catalyst is notbasically generated. However, this technique is carried out in areaction system in which the alcohol is used in excess and theconcentration calculated as triglyceride, is 0.1 to 3% by weight or so,so that the activity is drastically low, the reaction temperature isalso limited to 60° C. or lower from the viewpoint of the durability ofthe ion-exchange resin, and the like. Therefore, it cannot be said to bepractical.

Also, recently, use of a basic solid catalyst comprising a carboxylicacid compound and iron oxide, or a potassium compound and zirconiumoxide has been disclosed (Japanese Patent Laid-Open No. 2000-44984).However, its catalytic activity cannot be said to be satisfactory,thereby making it impractical.

In addition, in a case where the basic catalyst is used, other problemsare generated besides the problems as mentioned above. In other words,since a natural fat or oil generally contains a large amount of freefatty acids (3% by weight or more on average), if the basic catalyst isused, the side reaction of the formation of the fatty acid soap isdrastic, so that there arise some problems that the catalyst is requiredin excess, that the separation of the fatty acid ester layer and theglycerol layer becomes difficult due to the generated fatty acid soapand the like. Therefore, a pretreatment step of removing a free fattyacid would be necessitated.

From the viewpoint of avoiding the above problems, there has beendisclosed, for instance, in Japanese Patent Laid-Open No. Sho 61-14044,a process for converting a free fatty acid to an ester with an acidcatalyst as a pretreatment step. However, there arise some problems thatthe separation of the acid catalyst is difficult, and if the acidcatalyst remains in the reaction mixture when the transesterificationreaction is carried out, the basic catalyst (a metallic alkali catalyst)is undesirably neutralized, and thereby the amount of the solid catalystused is increased by the amount of the neutralized catalyst.

In addition, as a process for preparing an ester of a fatty acid notnecessitating the pretreatment step as mentioned above, there has beenproposed a process of using a solid acidic catalyst (for instance,Japanese Patent Laid-Open No. Hei 6-313188). However, the acidiccatalyst has a fatal defect that the activity for thetransesterification reaction of a fat or oil is low as compared to thatof the basic catalyst, so that there arise a problem that the catalystis required to be used in a large amount in the transesterificationreaction in which the acidic catalyst is used.

Furthermore, there has been known a technique of carrying out thetransesterification reaction under high-temperature and high-pressureconditions (240° C., 9 MPa) in the presence of a basic catalyst, therebyincreasing the reaction efficiency without requiring the pretreatment.Such a technique is described, for instance, in “JAOCS” (Vol. 61, No. 2,p. 343, 1984). However, since a homogeneously-used catalyst is used,there has been necessitated a purification step as an after-treatmentsuch as removal of the catalyst or removal of the fatty acid soappartially produced.

Moreover, as a process for preparing an alkyl ester of a fatty acid,there has been disclosed a process of carrying out transesterificationwithout a catalyst in an atmosphere in which an alcohol is made into asupercritical state (Japanese Patent Laid-Open Nos. 2000-109883 and2001-143586). However, its reaction rate is notably smaller as comparedto that of the homogeneously-used basic catalyst, and the reaction ratioat an equilibrium reached and the reaction rate are low under theconditions near the supercritical point, thereby making it almostimpractical. In order to improve the reaction rate, the conditions ofhigh-temperature and high-pressure must be made stricter. However, thereare some problems that the decomposition of the reaction product takesplace and that the reaction ratio is consequently lowered, and the like.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a process for preparingan alkyl ester of a fatty acid which can be utilized on an industrialscale, capable of preparing an alkyl ester of a fatty acid effectivelyutilized as a diesel fuel oil or the like mainly from a triglyceridecontained in a fat or oil, especially waste edible oil at a highreaction ratio under practical, relatively mild conditions. In thisprocess, there can be simplified or omitted a step of separating andcollecting the catalyst, or in addition to the step, a pretreatment stepof removing a free fatty acid contained in the fat or oil, and anafter-treatment step of removing a fatty acid soap.

Specifically, the gist of the present invention relates to:

-   [1] a process for preparing an alkyl ester of a fatty acid    characterized by a use of a catalyst comprising a composite metal    oxide having a perovskite structure in the preparation of the alkyl    ester of a fatty acid by a transesterification reaction between a    fat or oil and an alcohol in the presence of the catalyst; and-   [2] a process for preparing an alkyl ester of a fatty acid    characterized in that in the preparation of the alkyl ester of a    fatty acid by a transesterification reaction between a fat or oil    and an alcohol in the presence of a catalyst, the alcohol is made    into a supercritical state or subcritical state, and at least one    member selected from the group consisting of oxides, hydroxides and    carbonates of alkaline earth metals is used as the catalyst.

BEST MODE FOR CARRYING OUT THE INVENTION

A first invention of the present invention is directed to a process forpreparing an alkyl ester of a fatty acid, wherein a transesterificationreaction is carried out between a fat or oil and an alcohol in thepresence of a catalyst comprising a composite metal oxide having aperovskite structure; a second invention of the present invention isdirected to a process for preparing an alkyl ester of a fatty acid,wherein a transesterification reaction is carried out between a fat oroil and an alcohol in the presence of a catalyst comprising at least onemember selected from the group consisting of oxides, hydroxides andcarbonates of alkaline earth metals, with the alcohol made into asupercritical state or subcritical state.

One of the significant features of the first invention of the presentinvention resides in the use of a catalyst comprising a composite metaloxide having a perovskite structure. Since the catalyst is used, thetransesterification reaction between an ester (mainly a triglyceride)contained in a fat or oil and an alcohol can be carried out at a highefficiency even under milder conditions, thereby making it possible toprepare on an industrial scale a lower alkyl ester meeting therequirements of the Quality Assurance Regulation regarding a gas oil,which can be effectively utilized as a diesel fuel oil or the like,which had been conventionally difficult. Also, the catalyst is a solidand does not dissolve, for instance, in a reaction solution in which thetransesterification reaction is carried out. Since the catalyst can beeasily separated and removed from the reaction system with a simpleprocedure such as filtration after the termination of the reaction, thestep of separating and collecting the catalyst can be simplified oromitted. Therefore, an alkyl ester of a fatty acid can be easilypurified, and there would be little problem that the catalyst remains ina phase-separated glycerol, so that the resulting glycerol can beimmediately reused.

Here, the phrase “lower alkyl ester meeting the requirements of theQuality Assurance Regulation regarding a gas oil” concretely refers to alower alkyl ester having a sulfur content of 0.2% or less, a cetanenumber of 45 or more, and a temperature of 90% distillation of 360° C.or lower.

The fat or oil used as a raw material in the present invention is notparticularly limited. The fat or oil, for instance, includes naturalvegetable fats and oils and animal fats and oils, such as rapeseed oil,sesame oil, soybean oil, corn oil, sunflower oil, palm oil, palm kerneloil, coconut oil, safflower oil, linseed oil, cotton seed oil, tung oil,castor oil, tallow beef fat and pork fat, fish oil; waste edible oilsdisposed from restaurants, food factories and general households; andthe like. Also, these fats and oils can be used alone or in admixture oftwo or more kinds, and a fat- or oil-processed product comprising theabove fat or oil as a main component can also be used as a raw material.In the present invention, it is preferable to use a waste edible oilfrom the viewpoint of achieving the reuse of the resources.

The quality of the fat or oil used is not particularly limited. It ispreferable to use a fat or oil having low contents of water and a solidingredient, from the viewpoint of achieving the transesterificationreaction at a high efficiency. Therefore, when the waste edible oil isused as a raw material, it is preferable that the waste edible oil ispretreated in order to remove water and a solid ingredient containedtherein. Also, there may be some cases where an acidic substance iscontained in a large amount in a waste edible oil used. It is preferablethat the waste edible oil is subjected to deacidification as apretreatment from the viewpoint of preventing the inhibition of thecatalytic activity by the acidic substance. Here, any of the abovepretreatments can be carried out by a known method.

The alcohol used in the present invention is not particularly limited. Alower alcohol having a saturated, linear or branched hydrocarbonbackbone preferably having 1 to 8 carbon atoms, more preferably having 1to 5 carbon atoms is preferable, from the viewpoint of preparing ahigh-quality lower alkyl ester used as a diesel fuel oil. The alcoholincludes, for instance, methyl alcohol, ethyl alcohol, propyl alcohol,isopropyl alcohol, butyl alcohol, t-butyl alcohol and the like. Amongthem, at least one member selected from the group consisting of methylalcohol, ethyl alcohol, propyl alcohol and isopropyl alcohol is morepreferable, and methyl alcohol is still more preferable, from theviewpoints of low cost and facilitation in collection.

The catalyst used in the present invention is a catalyst comprising acomposite metal oxide having a perovskite structure, wherein thecomposite metal oxide has a composition represented by the generalformula: ABO_(n), wherein A is an alkaline earth metal atom, B is atransition metal atom, and n is an integer of 3 or more. From theviewpoint of exhibiting the desired effects of the present invention, Ais preferably at least one member selected from the group consisting ofCa, Sr and Ba, and B is preferably at least one member selected from thegroup consisting of Ti, Mn, V, Fe, Cu, Y and La, which is a lanthanidemetal atom. Further, as the above catalyst, more preferable are thosecomprising a composite metal oxide in which A is at least one memberselected from the group consisting of Ca, Sr and Ba, and B is at leastone member selected from the group consisting of Ti, Mn, Y and La. Here,the perovskite structure is a structure generally formed in a case whereA ion is notably large in the structure having a composition of ABO₃. Anoxide includes A₊₁B₊₅O₃, A₊₂B₊₄O₃, A₊₃B₊₃O₃, and the like. Thus, theperovskite structure refers to a structure of a composite perovskitecompound containing plural A or B ions, the structure of which isdetermined by X-ray diffraction.

In addition, as the above catalyst, those further comprising a cesium(Cs) compound are preferable. The Cs compound includes, for instance,Cs₂O, CsCl, CsF, Cs₂CO₃, CsNO₃, Cs₂SO₄, and the like. The content of thecompound is not particularly limited as long as the desired effects ofthe present invention are obtained, and is preferably from 1 to 10% bymass, more preferably from 3 to 8% by mass, as calculated by the mass ofcesium. Among them, as the catalyst comprising the Cs compound, thosecomprising a composite metal oxide in which A is at least one memberselected from the group consisting of Ca, Sr and Ba, and B is at leastone member selected from the group consisting of Ti, Mn, Y and La,together with the Cs compound are preferable.

It is considered that the catalyst comprising the composite metal oxidehaving a perovskite structure used in the present invention is a solid,strongly basic catalyst, in which the composite metal oxide is deducedto form a stable structure comprising oxides of a basic metal atomhaving a relatively larger radius such as alkaline earth metal atoms andlanthanide metal atoms; and since the composite metal oxide especiallyhas a perovskite structure, the basic metal oxide having a large ionicradius is structurally stabilized, so that structural change of thecatalyst is less likely to be caused when the catalyst is used in thereaction as a catalyst. In addition, although the compound itself havinga perovskite structure has a strong basicity, it is deduced that even ifa Cs-containing compound having a large ionic radius is further added tothe catalyst, the perovskite structure is stably maintained, therebyestablishing additive property of basicity of both compounds, so thatthe basicity of the catalyst is increased more by further addition ofthe Cs compound to the catalyst. Therefore, the catalyst forms amaterial system having remarkably large basic strength as the basicitywhich can be expected for the composite metal oxide, so that thecatalyst can be a catalyst having a more excellent activity. Therefore,according to the catalyst of the present invention, a high catalyticactivity is exhibited in the transesterification reaction, so that adesired lower alkyl ester which can be effectively utilized as a dieselfuel oil or the like can be obtained at a high efficiency, which hadbeen conventionally difficult. In addition, since the catalyst is asolid, the separation and collection of the catalyst after thetermination of the reaction are facilitated, and there is no limitationon the reaction temperature under ordinary esterification reactionconditions, which is different from that of the above known solid basiccatalyst made of an ion exchange resin having an amine-based base.

The preparation of the catalyst of the present invention is carried outby mixing and supporting each metal used as a raw material to each otherby coprecipitation method, impregnation method, kneading method or thelike, and baking the resulting mixture. Specifically, the preparationcan be carried out, for instance, by mixing aqueous solutions containingeach metal used as a raw material with stirring, aging the mixture,thereafter obtaining a precipitate, washing, filtering, drying andbaking the precipitate. Further, the catalyst obtained as a powder canbe molded to give a so-called “solid catalyst,” which is preferablebecause the possibility of admixing the catalyst into a reactionsolution can be dramatically reduced, and the step of separation andcollection of the catalyst after the termination of the reaction doesnot substantially have to be carried out.

In the preparation of the aqueous solution containing each metal used asa raw material, there can be used a carbonate, a nitrate, a sulfate, aformate, an acetate or the like, each containing A shown in the abovegeneral formula; and a carbonate, a nitrate, a sulfate, a formate, anacetate or the like, each containing B shown in the above generalformula. The concentration of the metal in the aqueous solution is notparticularly limited as long as the aqueous solution of the metal isobtained. Next, the aqueous solution containing each metal is mixed atpreferably from 30° to 60° C. with stirring. The mixing is carried outin the presence of an alkali metal, which acts as an agent forincreasing pH of the aqueous solution as a precipitating agent, and isnot contained in the catalyst, from the viewpoint of forming aprecipitate of a hydroxide having a uniform composition from each metalsalt. For instance, an aqueous solution of Na salt may be prepared, andmixed with an aqueous solution containing each of the above metals usedas a raw material. The mixing may be carried out by, for instance,supplying each of the resulting aqueous solutions at once in one vessel.It is preferable that an aqueous solution containing each metal used asa raw material is added dropwise against an aqueous solution containingan alkali metal, from the viewpoint of efficiently progressing theformation of the catalyst. It is desired in the mixing that the metal ofthe component A and the metal of the component B are added at a molarratio of preferably 1:1. Also, it is desired that the alkali metal isused so that a molar ratio between the alkali metal and a total of themetals of the components A and B is preferably from 1:1 to 5:1. Themethod of stirring is not particularly limited.

The aging refers to a step of growing the core formed during theprecipitation, which is preferably carried out at the same temperaturerange as that in mixing and stirring of the above aqueous solution for 1to 4 hours. By this step, since the solid particles of the formedcatalyst can be obtained as a precipitate, the precipitate is washed bywashing with water and filtration. After washing, the precipitate isproperly dried by, preferably air-drying at 30° to 80° C., and furtherbaked in a muffle furnace at preferably 500° C. or higher, morepreferably from 800° to 1000° C. for 1 to 4 hours.

In a case where the Cs compound is to be contained in the catalyst, itcan be carried out properly mixing a powder of a catalyst obtained afterdrying with a powder of a Cs compound, for instance, Cs₂O₃, Cs₂CO₃,CsNO₃, Cs₂SO₄, CsCl, CsCOOCH₃, Cs₂O, and the like, and thereafter bakingthe mixture in the same manner as mentioned above.

The catalyst can be molded by a known method. The term “molding” as usedherein also includes an embodiment where a powdery catalyst is supportedonto a specified carrier in addition to an embodiment where a powderycatalyst is molded into a given shape with a pressing machine or thelike.

The shape of the solid catalyst obtained by molding the catalyst of thepresent invention is not particularly limited, and any one can beappropriately selected in accordance with the application. The shape maybe any of, for instance, tablet form, ring form, pelletal form,honeycomb form and corrugated form, or those prepared by wash-coating aslurry containing a powdery catalyst onto a carrier made of ceramic, ametallic honeycomb or the like.

The process for preparing an alkyl ester of a fatty acid of the presentinvention may be a continuous process in which the transesterificationreaction is carried out by continuously feeding the raw materials, or abatch process in which the transesterification reaction is carried outonce by feeding the raw materials in an amount required for one batch.In a case of the continuous process, for instance, there can be carriedout the process by continuously feeding a fat or oil and an alcohol,which are raw materials, to a reactor for carrying out thetransesterification reaction, and discharging an alkyl ester of a fattyacid and glycerol, which are reaction products, and a residual fat oroil and alcohol, which are unreacted substances, wherein a fixed bed,flow-type reactor in which a solid catalyst prepared by molding thecatalyst (powder) of the present invention is filled and fixed is usedas the reactor. On the other hand, in a case of the batch process, therecan be carried out the process by introducing each of a fat or oil, analcohol and a catalyst, which are raw materials to a reactor used forcarrying out the transesterification reaction in an amount required forone batch to carry out the reaction, and taking out the reaction mixturecomprising an alkyl ester of a fatty acid and glycerol, which arereaction products, and a residual fat or oil and alcohol, which areunreacted substances. Among them, the embodiment of carrying out thepreparation of the alkyl ester of a fatty acid by carrying out thetransesterification reaction using a fixed bed, flow-type reactor, inwhich a solid catalyst prepared by molding the catalyst (powder) of thepresent invention is filled and fixed is preferable because the reactioncan be progressed in a high efficiency, and the step of separating andcollecting the catalyst can be also omitted after the termination of thereaction. In a case of the batch process, the catalyst can be alsoeasily removed by filtration or the like, so that the step of separatingand collecting the catalyst can be simplified.

The present invention will be explained hereinbelow for a case ofpreparing a lower alkyl ester using the above fixed bed, flow typereactor, which is a preferred embodiment as the process for preparing analkyl ester of a fatty acid of the present invention. Here, in thefollowing explanations, “parts” represents “parts by weight” unlessspecified otherwise.

The amount of the lower alcohol charged to the fat or oil is preferablyfrom 10 to 100 parts, preferably from 10 to 40 parts, based on 100 partsof the fat or oil. However, the amount of the lower alcohol charged to100 parts of the fat or oil, which is the raw material, is changeabledepending on an average molecular weight of the fat or oil. In the case,the amount charged can be approximately expressed as a multiple ofchemical equivalent which is calculated by the following equation (1).In other words, if the average molecular weight of the fat or oil isrepresented by Mo and the average molecular weight of the fat or oil isrepresented by Ma, the equivalent (Wa) charged of the lower alcohol iscalculated by the following equation:Wa=100/Mo×3Ma  (1)The lower alcohol is charged at a ratio of preferably from 1.2- to10-folds, more preferably from 1.5- to 5-folds equivalent Wa charged. Ifthe charging ratio of the alcohol is within the above range, thetransesterification reaction is sufficiently progressed, thereby makingit also preferable from the economic viewpoint. Here, the averagemolecular weight is each calculated on the basis of the composition ofthe fat or oil and the lower alcohol used as raw materials.

For instance, when the average molecular weight of the fat or oil is 880and the lower alcohol is methyl alcohol (average molecular weight: 32),methyl alcohol is charged in an amount of preferably from 12 to 100parts, more preferably from 15 to 50 parts, based on 100 parts of thefat or oil.

The amount of the solid catalyst charged to the reactor is notparticularly limited as long as the desired efficiency for thetransesterification reaction is obtained. Generally, the amount of thesolid catalyst charged is preferably from 0.5 to 100 parts, morepreferably from 1 to 30 parts, based on 100 parts of the fat or oil. Aslong as the amount of the solid catalyst charged is within the aboverange, the transesterification reaction is sufficiently progressed,thereby making it also preferable from the economic viewpoint.

The reactor, namely the vessel for providing a reaction site for thetransesterification reaction, includes flasks, steel vessels, steelpipes, static mixers, agitation vessels and the like. The materials ofthe reactor are not particularly limited, and there can be used glass,steel, stainless steel, Ni alloys, Ti alloys, glass lining steel,polymer lining steel, ceramic lining steel and the like.

The fat or oil and the lower alcohol may be fed to the reactor, forinstance, by separately feeding via raw material feeding lines connectedbetween raw material tanks and the reactor (embodiment 1); or previouslymixing a fat or oil and a lower alcohol in one raw material tank, andfeeding both the components simultaneously via one raw material feedingline (embodiment 2). The order of the charging of each raw material isnot particularly limited. In addition, the embodiment 2 is intended toencompasses all the embodiments in which both components are mixed andsimultaneously fed to the reactor. For instance, there may be includedan embodiment where the raw material tank is provided for each rawmaterial and the raw material feeding lines are united into one line, sothat a so-called line-mixing is carried out and both components aresimultaneously fed to the reactor.

In the case of the embodiment 1, the feed amount of the fat or oil tothe solid catalyst in the reactor is preferably from 0.1 to 10/hr interms of the liquid space velocity (LHSV). In addition, the feed amountof the lower alcohol is preferably such that the amount of the loweralcohol charged to the fat or oil is the above preferred range,preferably from 0.01 to 6/hr in terms of the liquid space velocity(LHSV).

In the case of the embodiment 2, the fat or oil and the lower alcoholare previously mixed so that the amount of the lower alcohol charged tothe fat or oil is within the above preferred range, and the resultingmixture is fed to the solid catalyst in the reactor preferably at 0.7 to8/hr in terms of the liquid space velocity (LHSV).

The liquid space velocity as used herein is a value obtained under theconditions of 25° C. and 1 atmosphere.

The transesterification reaction is carried out by heating each of thefed raw materials to a given reaction temperature in the reactor. Here,the method of heating may be charging each raw material with heating theraw material by using a heat exchanger during charging, or heating thereactor externally from the beginning of charging. The reactiontemperature is preferably from 25° to 300° C., more preferably from 40°to 200° C., still more preferably from 55° to 60° C. In the presentinvention, it is especially preferable to carry out the reaction at atemperature range of from 55° to 60° C. This is due to the fact that theabove temperature range is a very mild condition, and especially hasexcellent contribution to cost reductions of the equipments andfacilitation of the operation.

The reaction pressure corresponds to a vapor pressure at the reactiontemperature exhibited by a volatile substance charged in the reactor.The reaction pressure is preferably from 0.1 to 10 MPa, more preferablyfrom 0.1 to 6 MPa, still more preferably from 0.1 to 1 MPa. As long asthe reaction temperature and the reaction pressure are within the aboveranges, it is preferable because the rate for the transesterificationreaction is sufficient, so that the reaction can be excellently andrapidly progressed. The reaction time cannot be absolutely determinedbecause the reaction time differs depending upon the reactiontemperature and the kind of the raw materials used. The reaction time isgenerally selected within the range of from 1 minute to 24 hours.

The reaction product obtained by the transesterification reaction in thereactor is a mixture comprising as main components glycerol and a loweralkyl ester of a fatty acid formed by the transesterification reactionof a lower alcohol and a triglyceride contained as a main component inthe fat or oil used as a raw material. In order to separate the loweralkyl ester and glycerol from this mixture, there can be applied astand-still phase separation (decantation) method comprising cooling theresulting reaction mixture to a given temperature after the terminationof the reaction, allowing the mixture to stand at the above temperatureor at room temperature, or after the termination of the reaction,directly allowing the mixture to stand at room temperature, to causephase separation between the lower alkyl ester and the glycerolutilizing their difference in specific gravity. However, thecentrifugation method described below is preferable from the viewpointof productivity.

The centrifugation method is carried out by subjecting the reactionmixture cooled to a desired temperature after the termination of thereaction to a centrifugal separator, and applying a centrifugal force toa degree that the lower alkyl ester and the glycerol can be separated.In the upper layer obtained after the centrifugation, there arecontained admixtures of an unreacted alcohol component, an odorouscomponent, a colored component and the like besides the main componentlower alkyl ester. Therefore, if the admixtures are subsequently removedby a known purification step using, for instance, distillation oradsorbent agent, a lower alkyl ester can be obtained in a high purity.Here, the adsorbent agent which can be used in the purification stepincludes, for instance, activated charcoal, active carbon fiber,activated clay, acidic clay, bentonite, diatomaceous earth, activatedalumina and the like.

One of the significant features of the second invention resides in thata transesterification reaction between an ester (mainly a triglyceride)contained in a fat or oil and an alcohol is carried out in theco-presence of at least one member selected from the group consisting ofoxides, hydroxides and carbonates of alkaline earth metals as acatalyst, with the alcohol made into a supercritical state orsubcritical state. According to the present invention, thetransesterification reaction can be carried out at a high efficiencybetween the ester contained in a fat or oil and the alcohol underpractical, mild conditions as compared to the above supercriticalreaction system of an alcohol in the absence of a catalyst, so thatthere can be prepared on an industrial scale a lower alkyl ester meetingthe requirement of the Quality Assurance Regulation concerning a gasoil, which can be effectively utilized as a diesel fuel oil, which hadbeen conventionally difficult. In this invention, the reaction iscarried out in the presence of the above catalyst, with the alcohol madeinto a supercritical state or subcritical state; in other words, thealcohol maintained under high-temperature and high pressure conditionsto activate the alcohol. Therefore, as compared to the neutralizationreaction of a free fatty acid contained in the fat or oil with analkaline earth metal contained in the catalyst, the progress of theesterification reaction of the fatty acid with the alcohol would beprioritized. Therefore, a pretreatment step of esterifying or removingthe fatty acid and an after-treatment step of removing a fatty acid soapwould not be necessitated. Also, the catalyst is a solid and does notdissolve in, for instance, a reaction solution in which thetransesterification reaction is carried out. Therefore, the catalyst canbe easily separated and removed from the reaction system with a simpleprocedure such as filtration after the termination of the reaction, sothat the step of separating and collecting the catalyst can besimplified or omitted. Therefore, an alkyl ester of a fatty acid can beeasily purified and there would be little problem that the catalystremains in a phase-separated glycerol. The resulting glycerol can beimmediately reused.

The fat or oil used as a raw material in this invention is notparticularly limited. The fat or oil, for instance, includes thoseexemplified above. Like in the above first invention, it is preferableto use waste edible oil from the viewpoint of achieving reuse of theresources. In addition, the quality of the fat or oil used may be thesame as that of the above first invention. The above pretreatment may becarried out for removal of water or the like and deacidification asdesired. Also, the alcohol used in this invention may be the same asthose of the above first invention.

In this invention, there is used as a catalyst at least one memberselected from the group consisting of oxides, hydroxides and carbonatesof alkaline earth metals. The alkaline earth metal is preferably atleast one member selected from the group consisting of Mg, Ca, Sr andBa, more preferably Mg. Preferred examples of the catalyst include MgO,Mg(OH)₂, MgCO₃, CaO, Ca(OH)₂, CaCO₃, and the like.

Since the catalyst is a solid, the separation and collection of thecatalyst after the termination of the reaction are facilitated. Inaddition, the catalyst can be molded to give a so-called “solidcatalyst,” which is preferable because the possibility of admixing thecatalyst into the reaction solution can be dramatically reduced, and thestep of separation and collection of the catalyst after the terminationof the reaction does not substantially have to be carried out. The shapeof the solid catalyst includes, for instance, those exemplified above.

In the preparation of an alkyl ester of a fatty acid of the presentinvention, the transesterification reaction is carried out in theco-presence of at least one member selected from the group consisting ofoxides, hydroxides and carbonates of alkaline earth metals as acatalyst, with the alcohol made into a supercritical state orsubcritical state. So far, as mentioned above, there has been known theprocess for preparing of a fatty acid ester by carrying out atransesterification reaction in the absence of a catalyst with analcohol made into a supercritical state. However, this process could notbe satisfactory for practical uses from the viewpoints of reactionefficiency and reaction rate. On the other hand, the present inventionhas a constitution that the alcohol is activated by making the alcoholinto a supercritical state or subcritical state, and the activation ofthe alcohol and the specified catalyst are combined. In a case where analkali metal catalyst which has been conventionally used as a catalystfor transesterification reaction is used as the catalyst, since thealkali metal catalyst is strongly basic, a neutralization reaction withan acidic component, especially a free fatty acid, contained in the rawmaterial fat or oil, irreversibly takes place, so that a fatty acid soapis generated in a large amount, thereby causing worsening influences tothe step subsequent to the transesterification reaction. In other words,an ester-containing phase and a glycerol phase are emulsified, therebymaking it difficult to separate the phases. Also, the catalyst itselfhas to be used in a large amount. Therefore, in such a case, the desiredeffects of the present invention cannot be obtained. In the presentinvention, there are surprisingly exhibited excellent desired effects ofthe present invention which cannot be anticipated from those of theprior arts by using as the “specified catalyst” an oxide of an alkalineearth metal or the like, which has not been ordinarily used as acatalyst for transesterification reaction because of low efficiency ofthe reaction. In a case where the transesterification reaction iscarried out by combining the activation of the alcohol and the catalystcomprising an oxide of an alkaline earth metal or the like, it isdeduced that the basicity of the catalyst is weak, so that under theconditions that the alcohol is made into a supercritical state orsubcritical state, the free fatty acid is esterified before theneutralization reaction takes place with an acidic substance, especiallya free fatty acid, and so that further the neutralization reaction, ifcaused at all, is reversible, whereby the free fatty acid involved inthe neutralization reaction can be also esterified.

For instance, in a case where methyl alcohol is used as an alcohol, themethyl alcohol in a supercritical state or subcritical state isactivated by dissociating as follows:MeOH⇄MeO⁻+H⁺It is deduced that the proton (H⁺) generated by the dissociation ofmethyl alcohol activates the ester (mainly a triglyceride) contained inthe fat or oil together with the catalyst, an oxide of an alkaline earthmetal or the like, thereby effectively progressing thetransesterification reaction between the methyl alcohol and the ester.On the other hand, it is deduced that since the free fatty acidcontained in the fat or oil is immediately esterified by activatedmethyl alcohol, the neutralization reaction with the alkaline earthmetal contained in the catalyst is hardly likely to take place, and thateven if the neutralization reaction takes place, the product isdissociated again and the resulting free fatty acid is esterified.

The process for preparing an alkyl ester of a fatty acid of the presentinvention may be a batch process in which the transesterificationreaction is carried out once by feeding the raw materials in an amountrequired for one batch, or a continuous process in which thetransesterification reaction is carried out by continuously feeding theraw materials. In a case of the batch process, there can be carried outthe process by introducing each of a fat or oil, an alcohol and acatalyst, which are raw materials to a reactor used for carrying out thetransesterification reaction, in an amount required for one batch tocarry out the reaction, and taking out the reaction mixture comprisingan alkyl ester of a fatty acid and glycerol, which are reactionproducts, and a residual fat or oil and alcohol, which are unreactedsubstances. On the other hand, in a case of the continuous process,there can be carried out the process by continuously feeding a fat oroil and an alcohol, which are raw materials, to a reactor for carryingout the transesterification reaction, and discharging an alkyl ester ofa fatty acid and glycerol, which are reaction products, and a residualfat or oil and alcohol, which are unreacted substances, wherein a fixedbed, flow-type reactor in which a solid catalyst prepared by molding thecatalyst of the present invention is filled and fixed is used as thereactor. Among them, the embodiment of carrying out the preparation ofthe alkyl ester of a fatty acid by carrying out the transesterificationreaction using a fixed bed, flow-type reactor, in which a solid catalystprepared by molding the catalyst of the present invention is filled andfixed is preferable from the viewpoint that the control of the reactiontime under high-temperature and high-pressure conditions is furtherfacilitated in the process for preparing an alkyl ester of a fatty acidof this invention in which the transesterification reaction is carriedout with the alcohol maintained in a supercritical state or subcriticalstate. In addition, it is preferable because the reaction can beprogressed in a high efficiency, and the step of separating andcollecting the catalyst after the termination of the reaction can bealso omitted. In the case of the batch process, the catalyst can also beeasily removed by filtration or the like, so that the step of separatingand collecting the catalyst can be simplified.

The present invention will be explained hereinbelow for a case where theabove fixed bed, flow type reactor is used, which is a preferredembodiment as the process for preparing an alkyl ester of a fatty acidof the present invention. Here, in the following explanations, “parts”represents “parts by weight” unless specified otherwise.

The amount of the alcohol charged to the fat or oil is preferably fromabout 1.2- to about 50-folds, more preferably from about 1.2- to about30-folds stoichiometrically required amount to triglyceride contained inthe fat or oil as calculated on a molar basis. The amount is about1.2-folds or more from the viewpoint of excellently maintaining asupercritical state or subcritical state of the alcohol up to thetermination of the reaction, and the amount is about 50-folds or lessfrom the viewpoint of maintaining high volume efficiency of the reactionvessel and high reaction efficiency. Here, the stoichiometricallyrequired amount of the alcohol to the fat or oil is each calculated onthe basis of the component composition of the fat or oil and the loweralcohol used as the raw material.

The amount of the solid catalyst charged to the reactor is notparticularly limited as long as the desired efficiency for thetransesterification reaction is obtained. Generally, the amount of thesolid catalyst charged is preferably from 0.5 to 100 parts, morepreferably from 1 to 30 parts, based on 100 parts of the fat or oil. Aslong as the amount of the solid catalyst charged is within the aboverange, the transesterification reaction is sufficiently progressed,thereby making it also preferable from the economic viewpoint.

The reactor for carrying out the transesterification reaction and itsmaterial include, for instance, those exemplified above.

The embodiment for feeding the fat or oil and the alcohol to the reactorincludes, for instance, the above embodiment 1 and embodiment 2.

In this invention, in both cases of the embodiment 1 and the embodiment2, the feed amount of the fat or oil to the solid catalyst in thereactor is preferably from 1 to 30/h, more preferably from 1.5 to 20/h,in terms of the liquid space velocity (LHSV). The feed amount is 1/h ormore, from the viewpoint of the productivity per unit capacity of thereactor, and the feed amount is 30/h or less, from the viewpoint ofsufficiently carrying out the reaction in a high reaction rate.

On the other hand, the feed amount of the alcohol is preferably suchthat the amount of the alcohol charged to the fat or oil is the abovepreferred range, which can be determined depending upon the feed amountof the fat or oil.

The transesterification reaction is carried out in the reactor bycontrolling the pressure and the temperature so that the fed alcohol ismade into a supercritical state or subcritical state. The temperatureand the pressure at which the present invention is carried out maydiffer depending upon the kinds of the alcohol. For instance, in a casewhere methyl alcohol is used, the reaction temperature is usuallypreferably from 200° to 300° C., more preferably from 220° to 280° C.The reaction pressure corresponds to a vapor pressure at the reactiontemperature shown by a volatile substance charged in the reactor.Usually, the overall pressure inside the reactor is preferably from 3 to15 MPa, more preferably from 5 to 13 MPa. As long as the reactiontemperature and the reaction pressure are within the above ranges, it ispreferable because the alcohol is sufficiently activated, the thermaldecomposition of the substances involved in the reaction is suppressed,and also it is economically advantageous. Further, the balance betweenthe reaction temperature and the reaction pressure is important in thesufficient activation of the alcohol. From this viewpoint, thecombination of preferable reaction temperature and reaction pressureincludes the conditions where the reaction temperature is from 220° to280° C. and the reaction pressure is from 5 to 13 MPa, namely theconditions at which the alcohol is in a subcritical state. In thisinvention, it is especially preferable that the transesterificationreaction is carried out by making the alcohol in a subcritical statefrom the viewpoint of totally improving the productivity of the alkylester of a fatty acid especially in consideration of the cost reductionof the equipments and facilitation of the operation.

The reaction time cannot be absolutely determined because the reactiontime differs depending upon the reaction temperature and the kinds ofthe raw materials used and the like. The reaction time is generallyselected from the range of 1 minute to 24 hours. For instance, in a casewhere the reaction is carried out under temperature conditions exceeding250° C., the reaction time is preferably within 60 minutes, morepreferably within 40 minutes in order to suppress the thermaldecomposition of the glycerol or the like generated during the reaction.

Here, the method of heating the raw materials may be charging each rawmaterial with heating the raw material by using a heat exchanger duringcharging, or heating the reactor externally from the beginning ofcharging. Also, during the reaction, there may be some cases where afree fatty acid is partly adsorbed to an oxide of an alkaline earthmetal, which is the catalyst, in equilibrium, thereby generating thedeterioration of the catalytic activity. In such case, the regenerationof the catalyst can be readily carried out by feeding a hot air at ahigh temperature, preferably about 500° to about 600° C. to thecatalyst, to carry out oxidative decomposition of the adsorbed fattyacid.

The reaction product obtained by the transesterification reaction in thereactor is a mixture comprising as main components glycerol and an alkylester of a fatty acid formed by the transesterification reaction of analcohol and a triglyceride contained as a main component in the fat oroil used as raw materials. The separation of the alkyl ester of a fattyacid and glycerol from this mixture can be carried out in the samemanner as described above. In addition, the removal of the admixtures ofan unreacted alcohol component, an odorous component, a coloredcomponent and the like can be carried out in the same manner asdescribed above.

The alkyl ester of a fatty acid obtained by the process for preparing afatty acid ester of the present invention in the manner described abovehas a sufficiently high purity, and can be directly used as a gas oilsubstitute fuel or the like. Also, since glycerol does not contain thecatalyst, the glycerol can be immediately reused.

EXAMPLES

The constitution and the function and effects of the present inventionwill be specifically explained by means of Examples. The presentinvention is by no means limited by the examples, and any of modes withappropriate modifications within the range which can match the gistdescribed above and below are encompassed within the technical scope ofthe present invention.

Example 1-1

An aqueous solution prepared by dissolving 46 g of Na₂CO₃ in 540 ml ofwater was kept at 60° C. with stirring, and an aqueous solution preparedby dissolving 22.4 g of Ca(NO₃)₂.4H₂O and 28.3 g of TiOSO₄.nH₂O in 586ml of water was added dropwise thereto. Thereafter the mixture was agedfor 1 hour, to give a precipitate. The precipitate was filtered, washedwith water, and filtered, and thereafter the residue was dried, andbaked at 800° C. for 2 hours. An X-ray diffraction image of theresulting powder had a perovskite structure. Here, the X-ray diffractionwas carried out by RAD-IIB manufactured by Rigaku Denki. The bakedproduct thus obtained was molded into a tablet form with a hydraulicpressing molding machine, and the molded product was cut into 2 to 3 mmpieces, and the pieces were filled and fixed in the reactor as acatalyst. Subsequently, the reaction was started with feeding the rawmaterials. Here, the transesterification reaction was carried out underthe following reaction conditions.

(Reaction Conditions)

Raw material fat or oil: edible rapeseed oil+edible soybean oil(manufactured by THE NISSIN OIL MILLS, LTD.)

Raw material alcohol: methyl alcohol

Amount of catalyst: 10 ml (about 20 parts by weight based on 100 partsby weight of the fat or oil)

Feeding rate of fat or oil (LHSV): 2/h

Feeding rate of alcohol (LHSV): 0.26/h

Amount of alcohol charged: 13 parts by weight based on 100 parts byweight of the fat or oil

Reaction temperature: 60° C.

Reaction pressure: normal pressure (0.1 MPa)

After 1 hour passed from the beginning of the reaction, the generatedsolution was sampled every hour, and methyl alcohol was removed with anevaporator from the methyl ester phase of the sample, and thereafter theviscosity of the methyl ester solution was determined, and the ratio ofmethyl ester generated was obtained. The ratio of methyl ester generatedmeans the mass ratio (%) occupied with the methyl ester in the methylester phase (including triglyceride) of the generated solution fromwhich methyl alcohol was removed. Here, the determination of theviscosity was carried out with a viscosity analyzer [BL-type viscometer(manufactured by TOKIMEC, INC.)]. The amount of methyl ester in themethyl ester phase was obtained by preparing a calibration curve byobtaining viscosities of the methyl ester solutions of knownconcentrations, and determining the amount from the calibration curve.

The ratio (%) of methyl ester generated obtained by analyzing the sample3 hours after the beginning of the reaction is shown in Table 1.

Example 1-2

The same procedures as in Example 1-1 were carried out except that 28.6g of Na₂CO₃ was dissolved in 337 ml of water, and that 17.7 g ofCa(NO₃)₂.4H₂O and 21.5 g of Mn(NO₃)₂.6H₂O were dissolved in 273 ml ofwater, to prepare a catalyst. According to the X-ray diffraction, thecatalyst was found to have a perovskite structure. Thetransesterification reaction was carried out in the same manner as inExample 1-1, and the ratio of methyl ester generated at this time wasdetermined. The results are shown in Table 1.

Comparative Examples 1-1 and -2

The transesterification reaction was carried out in the same manner asin Example 1-1 except that one prepared by baking each of CaO and TiO₂at 800° C. was used as a catalyst, and the ratio of methyl estergenerated at this time was determined. As a result of the X-raydiffraction, it was confirmed that the catalyst did not have aperovskite structure. The results are shown in Table 1.

Example 1-3

In 405 ml of water was dissolved 33.3 g of Na₂CO₃, and 19.0 g ofSr(NO₃)₂ and 16.8 g of TiOSO₄.nH₂O were dissolved in 415 ml of water,and thereafter a precipitate was obtained in the same manner as inExample 1-1. The amount 18.2 g of a dried precipitate and 1.82 g ofCs₂O₃ were mixed, and then the resulting mixture was baked at 800° C.for 2 hours, and thereafter the catalyst was prepared in the same manneras in Example 1-1. According to the X-ray diffraction, the catalyst wasfound to have a perovskite structure. The transesterification reactionwas carried out in the same manner as in Example 1-1, and the ratio ofmethyl ester generated at this time was determined. The results areshown in Table 1.

Example 1-4

The catalyst was prepared in the same manner as in Example 1-3 exceptthat 49.8 g of Na₂CO₃ was dissolved in 588 ml of water, and that 28.15 gof La(NO₃)₃.6H₂O, 15.4 g of Ca(NO₃)₂.4H₂O and 31.4 g of Cu(NO₃)₂.3H₂Owere dissolved in 474 ml of water. According to the X-ray diffraction,the catalyst was found to have a perovskite structure. Thetransesterification reaction was carried out in the same manner as inExample 1-1, and the ratio of methyl ester generated at this time wasdetermined. The results are shown in Table 1.

Example 1-5

The catalyst was prepared in the same manner as in Example 1-3 exceptthat 25.4 g of Na₂CO₃ was dissolved in 300 ml of water, and that 9.6 gof Y(NO₃)₂.6H₂O, 13.1 g of Ba(NO₃)₂ and 24.2 g of Co(NO₃)₂.3H₂O weredissolved in 318 ml of water. According to the X-ray diffraction, thecatalyst was found to have a perovskite structure. Thetransesterification reaction was carried out in the same manner as inExample 1-1, and the ratio of methyl ester generated at this time wasdetermined. The results are shown in Table 1.

Example 1-6

The catalyst was prepared in the same manner as in Example 1-3 exceptthat 38.2 g of Na₂CO₃ was dissolved in 450 ml of water, and that 43.3 gof La(NO₃)₃.6H₂O, 21.2 g of Sr(NO₃)₂ and 29.1 g of Co(NO₃)₂.6H₂O weredissolved in 545 ml of water. According to the X-ray diffraction, thecatalyst was found to have a perovskite structure. Thetransesterification reaction was carried out in the same manner as inExample 1-1, and the ratio of methyl ester generated at this time wasdetermined. The results are shown in Table 1.

Example 1-7

The catalyst was prepared in the same manner as in Example 1-3 exceptthat 38.2 g of Na₂CO₃ was dissolved in 450 ml of water, and that 43.3 gof La(NO₃)₃.6H₂O, 21.2 g of Sr(NO₃)₂ and 40.4 g of Fe(NO₃)₃.9H₂O weredissolved in 545 ml of water. According to the X-ray diffraction, thecatalyst was found to have a perovskite structure. Thetransesterification reaction was carried out in the same manner as inExample 1-1, and the ratio of methyl ester generated at this time wasdetermined. The results are shown in Table 1.

Comparative Examples 1-3 and -4

The transesterification reaction was carried out in the same manner asin Example 1-1 except that one prepared by baking each of BaO and SrO at800° C. was used as a catalyst, and the ratio of methyl ester generatedat this time was determined. As a result of the X-ray diffraction, itwas confirmed that the catalyst did not have a perovskite structure. Theresults are shown in Table 1.

Comparative Example 1-5

The transesterification reaction was carried out in the same manner asin Example 1-1 except that one prepared by mixing 20 g of TiO₂ and 2 gof Cs₂CO₃, and baking the mixture at 800° C. was used as a catalyst, andthe ratio of methyl ester generated at this time was determined. As aresult of the X-ray diffraction, it was confirmed that the catalyst didnot have a perovskite structure. The results are shown in Table 1.

TABLE 1 Ratio of Methyl Ester Composition Generated of Catalyst (%) Ex.1-1 CaTiO₃ 92.1 Ex. 1-2 CaMnO₃ 88.0 Comp. Ex. 1-1 CaO 10.0 Comp. Ex. 1-2TiO₂ 0 Ex. 1-3 Cs₂O/SrTiO₂ 84.0 Ex. 1-4 Cs₂O/La₂CaCu₂O₆ 92.0 Ex. 1-5Cs₂O/YBa₂Cu₄O₈ 84.5 Ex. 1-6 Cs₂O/LaSrCoO₄ 88.3 Ex. 1-7 Cs₂O/LaSrFeO₃86.0 Comp. Ex. 1-3 BaO 0 Comp. Ex. 1-4 SrO 1.2 Comp. Ex. 1-5 Cs₂O/TiO₂20.8 *: All of the contents of the Cs compound in the catalysts ofExamples 1-3 to -7 and Comparative Example 1-5 were about 6% by masscalculated as the mass of Cs.

It can be seen from the comparisons of Examples 1-1 to -7 withComparative Examples 1-1 to -5 that the ratio of methyl ester generatedis remarkably improved by preparing a methyl ester using the catalystcomprising a composite metal oxide and having a perovskite structure, ascompared to a case where the catalyst is not used.

Example 2-1

An edible rapeseed oil and an edible soybean oil (both manufactured byTHE NISSIN OIL MILLS, LTD.) at a rate of 0.09 g per minute in total, andmethyl alcohol at a rate of 0.03 g per minute (about 3 times by mol ofthe stoichiometrically required amount) were continuously fed into astainless pipe having a diameter of 4.9 mm and a length of 250 mm inwhich 9.9 g of calcium oxide (CaO) molded into a grain of an about 1 mmwas filled and fixed (liquid space velocity: 1.3/h, calculated astriglyceride). The reaction temperature was kept at 280° C., and thereaction pressure was controlled to 6 MPa with an outlet control valve(the alcohol being in a subcritical state). The reaction productobtained after three hours from the beginning of the reaction wasseparated by centrifugation into an oily layer and a glycerol layer. Theratio of methyl ester of a fatty acid generated in the oily layer wasanalyzed by gas chromatography. As a result, the ratio was 98%.

Example 2-2

A reaction was carried out under the same conditions except thatmagnesium oxide (MgO) molded into a grain of about 1 mm was used. Theratio of methyl ester of a fatty acid generated after 3 hours was 98%.

Example 2-3

A reaction was carried out under the same conditions as in Example 2-1except that magnesium oxide (MgO) molded into a grain of about 1 mm wasused, and that unrefined palm oil having an acid value of 6 was used asa raw material oil. Samples were taken from the reaction mixture after 3hours and after 6 hours from the beginning of the reaction, and theratios of methyl ester of a fatty acid generated of the samples weredetermined in the same manner as in Example 2-1. The ratio generatedafter 3 hours was 94%, and the ratio generated after 6 hours was 96%.

Comparative Example 2-1

A reaction was carried out under the same conditions as in Example 2-1except that the catalyst was not filled. The ratio of methyl ester of afatty acid generated after 3 hours was 48%.

Comparative Example 2-2

A reaction was carried out under the same conditions as in ComparativeExample 2-1 except that the reaction temperature was 340° C. and thereaction pressure was 9 MPa (the alcohol being in a supercriticalstate). The ratio of methyl ester of a fatty acid generated after 3hours was 32%.

Comparative Example 2-3

A reaction was carried out under the same conditions as in Example 2-1except that the reaction temperature was 150° C. and the reactionpressure was 0.9 MPa (the alcohol being in neither a supercritical nor asubcritical state), and that MgO was used as the catalyst. The ratio ofmethyl ester of a fatty acid generated after 3 hours was 5%.

Comparative Example 2-4

A reaction was carried out under the same conditions as in Example 2-1except that a commercially available sodium hydroxide pellet (grain ofabout 5 mm) was used as the catalyst. As a result, the catalyst wasdissolved, so that an oily layer containing an ester and a glycerollayer were emulsified, thereby making it difficult to separate into thelayers.

It can be seen from Examples 2-1 to -3 that according to the process forpreparing an alkyl ester of a fatty acid of the present invention, themethyl ester of a fatty acid can be efficiently prepared. On the otherhand, it can be seen from Comparative Examples 2-1 and -2 that when thecatalyst of the present invention is not used, the preparationefficiency for the methyl ester of a fatty acid is dramatically loweredeven if the reaction temperature and the reaction pressure wereincreased. It can be seen from Comparative Example 2-3 that when thealcohol is neither in a supercritical state nor in a subcritical state,the preparation efficiency for the methyl ester of a fatty acid isdramatically lowered even if the catalyst of the present invention wereused. Also, it can be seen from Comparative Example 2-4 that when thealcohol is in a supercritical or subcritical state and an alkali metalcatalyst is used as a catalyst, the methyl ester of a fatty acid cannotbe easily separated, so that the preparation efficiency for the methylester of a fatty acid is dramatically lowered. In other words, it can beseen that even if any one of the required elements of the presentinvention were lacking, the preparation efficiency for the methyl esterof a fatty acid is dramatically lowered.

INDUSTRIAL APPLICABILITY

According to the present invention, the alkyl ester of a fatty acidwhich can be effectively utilized as a diesel fuel oil or the like canbe prepared at high efficiency and on an industrial scale from a fat oroil, especially triglyceride mainly contained in a waste oil. In theprocess for preparing the alkyl ester of a fatty acid of the presentinvention, the step of separating and collecting a catalyst, or inaddition to such a step a pretreatment step for removal of a free fattyacid contained in a fat or oil and an after-treatment step for removalof fatty acid soap can be simplified or omitted. Therefore, according tothe process, the productivity for the alkyl ester of a fatty acid isremarkably improved as compared to those of conventional processes.

1. A process for preparing an alkyl ester of a fatty acid characterizedby a use of a catalyst comprising a composite metal oxide having aperovskite structure in the preparation of the alkyl ester of a fattyacid by a transesterification reaction between a fat or oil and analcohol in the presence of the catalyst.
 2. The process according toclaim 1, wherein the composite metal oxide comprises at least one memberselected from the group consisting of Ca, St and Ba.
 3. The processaccording to claim 1 or 2, wherein the catalyst further comprises acesium compound in an amount of from 1 to 10% by mass as calculated bythe mass of cesium.
 4. The process of claim 1, wherein thetransesterification reaction is carried out by using a fixed bed,flow-type reactor in which a solid catalyst formed by molding thecatalyst is filled and fixed.
 5. A process for preparing an alkyl esterof a fatty acid characterized in that in the preparation of the alkylester of a fatty acid by transesterification reaction between a fat oroil and an alcohol in the presence of a catalyst, the alcohol is madeinto a supercritical state or subcritical state, and at least one memberselected from the group consisting of oxides, hydroxides, and carbonatesof alkaline earth metals is used as the catalyst.
 6. The processaccording to claim 5, wherein the alkaline earth metal is magnesium. 7.The process according to claim 5 or 6, wherein the transesterificationreaction is carried out by using a fixed bed, flow-type reactor in whicha solid catalyst formed by molding the catalyst is filled and fixed. 8.The process according to claim 4, wherein a feed amount of the fat oroil is from 1 to 30/h in terms of the liquid space velocity calculatedas a triglyceride.
 9. The process according to claim 1, wherein thealcohol is an alcohol having a saturated, linear or branched hydrocarbonbackbone having 1 to 8 carbon atoms.